The invention relates to a process for treatment of a feedstock that comprises hydrocarbons with at least four carbon atoms per molecule, greatly unsaturated. It pertains in particular to the purification of an olefinic fraction that contains butadiene in large part, in particular butadiene 1,3, a highly upgradable product as a raw material of elastomers.
The prior art is illustrated by Patent Applications WO-97 24413 and EP-A-0 273 900.
This fraction also contains impurities of acetylenes, vinylacetylene (VAC), and ethylacetylene (ETAC), generally at a height of 1 to 2%, which create flaws in the polymerization processes due to the presence of gums that these compounds have a tendency to create and that should therefore be eliminated.
To eliminate the acetylene compounds, it is known to hydrogenate them in the presence of a catalyst that contains a noble metal of group VIII of the periodic table, alone or with promoters, deposited on a substrate such as alumina or silica.
Thus, in the main processes for hydrogenation of the acetylene compounds, oligomers (primarily dimers and trimers) form in the hydrogenation reactor that must then be separated from butadiene.
The hydrogenation reactor can be installed upstream from the debutanization column. Under these conditions, the ratio of acetylene compositions to butadienes is very small in the feedstock and the selective hydrogenation of these compounds is difficult to carry out, causing relatively high losses of butadiene.
According to another variant of the prior art, the feedstock that is to be treated can be introduced in a debutanizer. The C4 compounds and acetylene compounds are recovered at the top of the debutanizer, then hydrogenated in a reactor. The hydrogenation effluent is distilled again in another distillation column. The purified C4 fraction is recovered at the top of the distillation column while the oligomers that are obtained at the bottom of said column are recovered. Indeed, the acetylene compounds were concentrated in the debutanizer since the C5 were separated at the bottom, but the overall process requires an additional distillation column, which increases the investment and operating costs. In addition, the ratio of acetylenes to butadienes is unchanged relative to the preceding variant.
Furthermore, Patent U.S. Pat. No. 5,866,734 describes a process for hydrogenation of an olefinic C4 fraction that contains, for the most part, butadiene and acetylene compounds so as to hydrogenate essentially completely the compounds with multiple double bonds and triple bonds without loss of unsaturated hydrocarbons with a double bond, whereby the reaction can take place in a catalytic distillation column.
One of the objects of the invention is to eliminate the drawbacks of the prior art.
Another object is to hydrogenate selectively the acetylene compounds that are contained in a C4 fraction in the presence of hydrogen without thereby causing losses of butadiene that are too significant and at a cost that is the lowest possible. Another object is to purify a fraction that is very high in butadiene while minimizing to the maximum the losses of butadiene that are linked to the distillation and hydrogenation of said fraction.
Specifically, the invention relates to a process for treatment of a feedstock that comprises hydrocarbons with at least four carbon atoms per molecule, whereby said feedstock comprises diene compounds and primarily butadiene as well as acetylene compounds in a minor proportion, whereby said process comprises a distillation stage of the feedstock introduced in a distillation zone that comprises a rectification zone and a drainage zone and at least one hydrogenation stage of acetylene compounds in at least one hydrogenation zone with at least one catalytic bed under suitable hydrogenation conditions in the presence of a gas that contains hydrogen, whereby the process is characterized in that a portion of the feedstock that circulates in the distillation zone that is enriched with acetylene compounds is drawn off laterally in liquid phase at a suitable draw-off level in the distillation zone and preferably in the drainage zone; the hydrogenation stage is carried out in the hydrogenation zone that is outside the distillation zone; a hydrogenation effluent that is low in acetylene compounds and enriched in oligomers is produced; and said hydrogenation effluent is recycled in the rectification zone, whereby the process is also characterized in that a C4 fraction that comprises essentially all of the butadiene and that is low in acetylene compounds is recovered at the top of the distillation zone, and an oligomer-enriched C5 fraction is recovered at the bottom of the distillation zone.
The rectification zone (also known as an enriching zone) is defined as a zone that is located above the feed level of the feedstock of the distillation column.
The drainage zone (also known as a stripping zone) is defined as a zone that is located below the feed level of the feedstock of the distillation column.
The feedstock can be a steam-cracking effluent that for the most part contains hydrocarbons with four to five carbon atoms per molecule and preferably a majority of hydrocarbons with four carbon atoms.
This feedstock according to the invention can contain at least 20% by weight of butadienes and preferably at least 50% by weight in the C4 fraction alone. Furthermore, it generally contains at most 20% by weight of acetylene compounds, advantageously at most 5% and preferably at most 2.5% by weight.
According to a characteristic of the invention, the draw-off flow can be at most equal to twice that of the feedstock that is introduced in the column, advantageously at most equal to 1.5 times the one of the column. It is by drawing off from the drainage zone a liquid fluid flow that is approximately equal to the one of the feedstock that is introduced into the column that the best results are obtained.
According to a particularly advantageous characteristic of the process, the feedstock can be introduced at a level that corresponds approximately to the center of the distillation column, the lateral draw-off level is located below said center of the column at a height that generally corresponds to fewer than five theoretical plates from said center, and the hydrogenation effluent is recycled above the center of the column at a level that generally corresponds to a height within the first five theoretical plates from the top of the column.
By suitable chromatographic measurements, the ratio of concentrations of acetylene compounds to butadiene can be determined, and the fluid can be drawn off laterally when this ratio, on the plate of the column that is being considered, is essentially the highest and advantageously higher than the one of the feedstock.
At the level of the reinjection of the product of the hydrogenation reaction, it is advantageous to reinject this flow the highest possible in the column relative to the draw-off level so as to create an internal reflux in the column and therefore to increase its power of separation.
The increase in temperature linked to the hydrogenation is generally small because the amount of hydrogenated products is very small. Nevertheless, it may be advantageous to control the exothermicity of the hydrogenation stage and the temperature of the hydrogenation effluent upstream from the recycling level in the rectification zone of the column; it is especially preferable to reintroduce the fluid at the top of the column at a temperature that is approximately equal to that of the reintroduction plate so as not to disturb the distillation column.
The operating conditions of the distillation column are usually as follows:
Number of theoretical plates 40, preferably 35-45
Absolute pressure 4-10 bar (1 bar=105 Pa)
Top temperature: 45xc2x0 C., preferably 30xc2x0 C. to 50xc2x0 C.
Bottom temperature: 95xc2x0 C., preferably 90xc2x0 C. to 150xc2x0 C.
The hydrogenation reactor is generally operated under the following conditions:
Absolute pressure: 2 to 70 bar (1 bar=105 Pa), preferably 5 to 15 bar
Temperature: 30 to 60xc2x0 C., preferably 35 to 45xc2x0 C.
Volumetric flow rate 3 to 10 311, preferably 4 to 8 hxe2x88x921 
The volumetric flow rate represents the catalyst volume divided by the liquid volume of fresh feedstock measured at 15xc2x0 C.
Ratio of H2/acetylene compounds (mol/mol): 0.5-3, preferably 1.0 to 1.1
Catalyst: either nickel or a collection mass that contains copper, or a noble metal of group VIII, preferably palladium, stabilized by at least one metal Au, Ag, for example 0.01 to 1% by weight of metal relative to the total weight of the catalyst;
said ratio of H2/acetylene compounds will be adjusted based on the required specifications for the top effluent of the distillation column.
The invention will be better understood based on the following FIGURE, illustrating a preferred embodiment in a diagrammatic form.
A hydrocarbon feedstock 1 that comprises a C4 fraction that is obtained from a steam-cracking device and that contains about 50% of butadienes and 1 to 2% relative to the C4 fraction alone of acetylene compounds is introduced into a distillation column 2 called a debutanizer. This column comprising about 40 theoretical plates, the feedstock is introduced at the level of the 20th plate. At the top of the column, the C4 fraction that contains butadiene and about 1000 ppm of acetylene compounds is recovered via a line 3. A portion of this fraction is reintroduced after condensation 20 and separation 21 in the form of reflux 5 at the top of the column while the other portion is recovered via a line 6 for a subsequent treatment, an extraction by solvent for example.
At the level of the 23rd plate of the column, a liquid fluid whose ratio of acetylene compounds/butadiene concentrations is approximately the highest of the column is drawn off laterally via a line 7 by taking into account the dilution by a factor of 2 by the product of the reaction, for example equal to 0.027 mol/mol.
This fluid is introduced into at least one hydrogenation reactor 8 that is fed with hydrogen via a line 9 under partial pressure conditions that essentially correspond to the stoichiometry of the hydrogenation of acetylene compounds. This reactor contains a downflow fixed bed (introduction via the top of the reactor of the liquid feedstock) of hydrogenation catalyst that can be palladium that is stabilized by gold on a gamma alumina substrate. The hydrogenation effluent is collected via a line 10, cooled in an exchanger 11 and recycled in the 4th theoretical plate, for example of the rectification zone of the column, at a temperature that is approximately equal to that of this 4th plate.
This effluent contains in particular the olefinic compounds that are present initially in the feedstock, the butadienes that essentially have not been hydrogenated as well as the oligomers produced in the hydrogenation zone. These oligomer compounds, which are heavy products, are collected at the bottom of the column via a line 13 as well as hydrocarbons with 5 carbon atoms per molecule of the feedstock. A portion is used to be introduced into a reboiler 14 and recycled at the bottom of the column via a line 15.
The following examples illustrate the invention.